In commercial vehicles such as, for example trucks and buses, use is often made of automatic or semiautomatic gear boxes. Such gear boxes can be likened to conventional manual gear boxes, with the difference that the shifting of gears is carried out by means of operating devices instead of manually by the driver.
Especially in heavy cargo vehicles, automatic gear boxes of the automatic increment shifted transmission kind are utilized. These have become more and more common as microcomputer technology has developed. It is now possible to, with the use of a control computer and a number of control devices, for example servo engines, precisely regulate the rpm of the engine, the engagement and disengagement of a disc coupling between the engine and the gear box, and the coupling members of the gear box relative to each other so that a smooth gear shift is always obtained at the proper rpm. One advantage with this kind of automatic gear box (as compared to a traditional automatic gear box with planetary gear steps and with a hydrodynamic torque converter on the inside) as regards heavy vehicles, is that it is simpler and more robust and can be produced at essentially lower costs than the traditional automatic gear box, and also, it has a higher degree of efficiency, which offers a possibility for lower fuel consumption.
The automatic gear box built on planetary gearing usually has one-way couplings between the planetary gear steps, which in the case of a driving engine in the automatic gear position will prevent torque transfer from the engine to the driving wheels, but which when transferring torque in the other direction; i.e., at zero gas and with the vehicle moving, disengages the gears and allows the engine to roll freely without engine braking, which brings about a lower fuel consumption by the use of the kinetic and position energy of the vehicle as compared to the case where the engine brakes. A corresponding free wheel function of present variable gear boxes has so far been impossible to obtain in another manner than through manual disengagement of the disc coupling between the engine and the gear box.
There is thus a desire to obtain a free wheel function in an automated increment shifted transmission which corresponds to the free wheel function of a traditional automatic gear box with planetary gear steps and free wheel couplings. This desire is generally based on the need to, as far as possible, limit the fuel consumption of the engine.
U.S. Pat. No. 4,078,631 shows an arrangement in a vehicle comprising (including, but not limited to) a gear box with a mechanical free wheel device which can be automatically activated when the vehicle has reached a pre-set velocity in order to thus reduce the fuel consumption of the engine. In more detail, this previously known arrangement is configured so that the engine is turned off electronically when a certain pre-set velocity has been reached. A free wheel function is then activated by means of a separate mechanical decoupling which decouples the engine from the gear box automatically when the positive; i.e., the driving torque from the engine approaches zero.
A drawback of this previously known arrangement is that a possibly desired engine braking will not be possible since the engine is decoupled from the gear box. Also, this previously known system does not take into account variations regarding outer driving conditions, such as for example the slope of the road, possible wind against or with the vehicle, aerial resistance, rolling resistance, and similar effects.